Add an example that supports gamepad/keyboard based navigation

[ajusa]

This was briefly discussed on the discord, but it would be very helpful to have a good example that shows how to extend Clay to support gamepad/keyboard based navigation in a UI.

I'm imagining something similar to https://www.youtube.com/watch?v=DYWTw19_8r4, but with arrow keys/d-pad allowing for navigation between the top menu, the bottom panel, and the left panel, and supporting scrolling with the d-pad. Hopefully this is possible to do with Clay as is - if it isn't, it may be helpful to extend the API to declare "focusable" elements somehow.